Which ANN structure to use?
If I understand your query correctly, you want to create a latent space that groups similar objects. You should then probably look for Siamese networks. However, your loss function will need another term to increase dissimilarity between different labels. Otherwise, as pointed out by Mike NZ, the net would collapse(yes, it is possible). Perhaps this will give some insights.
Note that the above method is not completely unsupervised. There are, in fact, a few claims of unsupervised classifications via clustering, although your objective function would look very different. You could go through this paper(called SCAN) for more details.
Hope it helps.
[Edit]
If you want a (lower-dimensional)representation of objects themselves, browsing through this could help. For a complex problem, linear reductions like PCA, although helpful, aren't probably what you're looking for. Here you can try training autoencoders. The loss function would work, along with some regularization term.